A negative-type photosensitive resin composition typically comprises an alkali-soluble resin, a polymerizable compound having at least one ethylenically unsaturated bond, a photopolymerization initiator and a solvent. The typical negative-type photosensitive resin composition is generally patterned by applying the resin composition to a substrate to form a film, irradiating a specific portion of the film with light through a photomask, and developing the exposed film to remove the unexposed portion of the film.
Such photosensitive resin compositions are used in various applications, including photosensitizing materials for the production of color filters, overcoat photosensitizing materials, column spacers, light-shielding insulating materials, etc.
With the recent increasing demand for liquid crystal displays (LCDs) in desktop computers and LCD TVs as well as notebook computers, LCDs are required to realize high quality colors. To comply with this requirement, pigment concentrations of photosensitizing materials for color filters are gradually increasing with finer dispersion of pigment particles. However, the developability of the photosensitizing materials tends to be poor. Further, there is a growing tendency to reduce the developing time of photosensitive resin composition for the production of color filters substrate to improve the production yield per unit time. Under these circumstances, an improvement in the developability of photosensitive resin compositions is needed.
A representative approach to improve the developability of photosensitive resin compositions is to increase the acid value (AV) of alkali-soluble resins as constituent binder resins of the photosensitive resin compositions. Another approach is to decrease the molecular weight (Mw) of the alkali-soluble resins.
On the other hand, a typical photosensitive resin composition comprises an alkali-soluble resin as a binder resin that is prepared by radical polymerization of an acid group-containing monomer and an ethylenically unsaturated monomer in the presence of a chain transfer agent (CTA). The chain transfer agent serves to control the molecular weight of the final resin. A mercaptan compound is most widely used as the chain transfer agent.
Recent issues encountered in the production of LCD color filter substrates are the formation of stepped portions of color patterns on black matrixes (BMs) and the use of large amounts of photoresists.
FIG. 1 is a partially enlarged cross-sectional view of a color filter substrate for an LCD. As shown in FIG. 1, the color filter comprises a black matrix 200 formed on a substrate 100 and a color filter layer 300 covering a portion of the black matrix 200. A stepped portion ‘A’ is formed along the edge of the color filter layer 300 due to the step height of the black matrix 200.
Thereafter, an alignment layer is formed on the color filter substrate and rubbing is performed. At this time, the stepped portion is insufficiently rubbed to cause a deterioration in the image quality (e.g., an increase in black luminance and a reduction in color contrast) of the LCD. In order to reduce the height (i.e. the difference in height between the distal end and the center of the pattern) of the stepped portion while maintaining a predetermined thickness of the pattern, it is necessary to lower the viscosity of a photosensitive resin composition for the formation of the color filter layer.
Thus, the content of a relatively viscous crosslinking agent in the photosensitive resin composition can be lowered to reduce the height of the stepped portion. Alternatively, the height of the stepped portion can be reduced by decreasing the molecular weight of a constituent binder resin to lower the viscosity of the photosensitive resin composition. However, the former method has the disadvantage of poor sensitivity despite a substantial reduction in the viscosity of the composition. According to the latter method, the composition may suffer from the disadvantages of poor chemical resistance and pattern peeling upon development. Further, the height of the stepped portion can be reduced by lowering the acid value of a constituent binder resin, but this is undesirable in terms of developability.
The photoresist must be used in reduced amounts with a high solids content to achieve target color coordinates. In this case, an increase in the viscosity of the composition is inevitably caused to make the coating thick, resulting in color distortion.
Under these circumstances, there is an urgent need to develop a photosensitive resin composition whose viscosity is sufficiently low so as to reduce the height of a stepped portion of a color pattern on a black matrix in a small amount.